// Package coast is what happens where the land meets the sea.
//
// Until this pass existed the coastline was only a *line*: the continent mask said which cells were ocean,
// the fluvial solve held those cells at sea level as its base level, and afterwards the sea floor was dropped
// to a flat plane 180 m down in one step. That is enough to give the solve a well-posed boundary — which is
// why D-48 kept the continent — and it is not a coast. There was no shelf, so a third of the map was a flat
// plane occupying a third of the elevation range; there was no surf, so the land met the water at whatever
// angle the last erosion step happened to leave it; and there was no sediment, so every bay was as deep as
// every headland was steep.
//
// # The three things this pass adds, and why each is a process rather than a shape
//
//  1. The shelf. A continental margin is a shelf at a very gentle grade out to a shelf break and then a much
//     steeper slope to the abyssal floor. The width of the shelf is not a constant: it is wide off a low
//     coastal plain and narrow where a range comes down to the water. So it is read off the relief standing
//     behind each stretch of shore rather than set, and the same manifest numbers then produce a wide shelf
//     on a passive coast and a narrow one on an active coast without either having been asked for.
//
//  2. The surf. Within a reach of the waterline the land is planed towards a shore platform. The reach is set
//     by how open the water is, so an exposed headland is attacked further inland than the back of a bay. The
//     cliff is not drawn: it is the step where the reach ends, and its height is whatever the land behind it
//     happened to stand at. That is the right way round — a sea cliff is tall because the land is tall, not
//     because a constant says so.
//
//  3. The sediment. What the surf cuts is counted, carried along the shore, and laid down in sheltered water
//     shallower than a few tens of metres: beaches and bars in the bays, nothing on the headlands. Rivers
//     deliver their own load at their mouths in proportion to what they drain, which is what makes a delta.
//     Mass is conserved to within the drift kernel's edges, and what will not fit under the berm is reported
//     rather than quietly dropped.
//
// # Why it runs after the solve and not before
//
// Two of the three need the finished terrain: the shelf width is a function of the relief behind the shore,
// and the surf cuts into whatever the solve built. The third could run before but would then be erased. So
// this pass owns the sea floor outright — uplift.Build no longer produces a bathymetry field — and it is the
// last thing that touches the geology grid.
//
// The one invariant it must not break: the sea floor is laid after the solve, never during it. A coastal cell
// drains into an ocean cell, and if that ocean cell sits at -180 m then the solver cuts the river down to
// -180 m; the first run with a coast eroded the land to 174 m below sea level for exactly that reason. A
// river's base level is sea level, and what the sea floor does below that is scenery.
package coast

import (
	"fmt"
	"math"
	"sort"

	"salty/terrain/internal/field"
	"salty/terrain/internal/manifest"
	"salty/terrain/internal/noise"
)

// srcShelf is this pass's noise stream. Pass indices are fixed and never reordered so that inserting a pass
// does not reshuffle the ones before it; uplift owns 1 to 9, so the coast starts at 10.
const srcShelf = 10

// backshoreM is how far inland the shelf looks for the relief that decides its width. Not a manifest key: it
// is the length over which "the land behind this beach" means anything, and 600 m is one hillside.
const backshoreM = 600

// The two anchors of the fetch scale, in fractions of the fetch range the seaward rays travelled. They are
// properties of how fetch is measured rather than of a world, which is why they are constants and not keys:
// a coast whose seaward rays nearly all run to the horizon is open however big the map is.
const (
	shelteredFetch = 0.35
	openFetch      = 0.90
)

// shelfSmoothM is how far the carried shelf width is smoothed along the coast. See shelfWidth.
const shelfSmoothM = 700

// exposureSmoothM is how far the carried fetch is smoothed along the coast. Shorter than the shelf's, because
// exposure is only read within a few hundred metres of the water and smoothing it over more than the reach of
// the processes that use it would flatten the very contrast it exists to provide.
const exposureSmoothM = 250

// shelterFloor is how much sediment the most exposed water will still take.
//
// Shelter cannot be a gate. Measured on the real continent with no floor, 73 % of the sediment budget came
// back unplaced, because the seaward rays from an ordinary stretch of coast nearly all run to the horizon and
// it therefore scores as fully exposed — and a fully exposed coast with no floor wants nothing at all. Real
// exposed coasts do have beaches; what they do not have is *more* sand than the bay next door. So the floor
// keeps the contrast, which is the part that matters, and stops the budget falling on the floor.
const shelterFloor = 0.15

// platformResidualCapM bounds what CutFraction may leave standing on the shore platform.
//
// CutFraction below 1 exists so the platform is not glass, and the obvious reading — leave that fraction of
// the height above the target — is wrong in a way that only shows on a tall coast: 15 % of a 120 m headland is
// 18 m, which is not a rough platform, it is an uncut headland. The residual is therefore a few metres at
// most, whatever the coast behind it stands at.
const platformResidualCapM = 4

// Input is everything the pass needs. Height is modified in place.
type Input struct {
	Height    *field.Field
	Sea       []bool  // the continent mask's ocean: the cells the solve held at base level
	SeaLevelM float64 // the base level the solve used, and the datum every depth here is measured from
	BreakM    float64 // depth at the shelf break, positive metres
	AbyssM    float64 // depth of the abyssal floor, positive metres
	Flow      []float32
	Seed      int64
	Cfg       manifest.Coast
}

// Result is the geometry the pass built and the accounting it kept.
type Result struct {
	Geometry *Geometry
	Exposure *field.Field // 0 sheltered, 1 open water; defined on every cell through Geometry.Ref
	Change   *field.Field // metres this pass moved: negative where the surf cut, positive where it laid
	Sea      []bool       // the mask as it now stands: everything strictly below sea level
	Stats    Stats
}

// Stats is the pass's own report. The volumes are the point: the sediment budget is the one part of this that
// is not derived from something already measured, so it is printed rather than assumed.
type Stats struct {
	ShorelineKm float64 `json:"shoreline_km"`
	SeaFraction float64 `json:"sea_fraction"`
	ShelfPctSea float64 `json:"shelf_pct_of_sea"`
	CutM3       float64 `json:"surf_cut_m3"`
	RiverM3     float64 `json:"river_load_m3"`
	LaidM3      float64 `json:"laid_m3"`
	UnplacedM3  float64 `json:"unplaced_m3"`
	RiverMouths int     `json:"river_mouths"`
	PlanedKm2   float64 `json:"planed_km2"`
	BeachKm2    float64 `json:"beach_km2"`
	DrownedKm2  float64 `json:"drowned_km2"`
	// How high the land stands immediately behind the surf strip, which is the cliff when there is one.
	//
	// The first version of this measured the drop from a cell to its seaward neighbour and called that the
	// cliff. That is a gradient, not a height: at the angle of repose one cell of a 10 m grid is 7 m, so the
	// number could not exceed 7 whatever the coast did, and it read 2 m on a plain coast and 3 m on a coast
	// with genuine cliffs on it. A cliff is how far you fall, not how steep the first cell is.
	BackshoreM    float64 `json:"backshore_m"`
	BackshoreP90M float64 `json:"backshore_p90_m"`
	// Exposure percentiles over the waterline cells, which is where it is measured and the only place it
	// means anything. A coast that is all 1.0 has no bays as far as the fetch can tell, and then neither the
	// surf reach nor the shelter is doing any work; a coast that is all 0 means the anchors are wrong. This
	// is the diagnostic to read before touching either.
	ExposureP10 float64 `json:"exposure_p10"`
	ExposureP50 float64 `json:"exposure_p50"`
	ExposureP90 float64 `json:"exposure_p90"`
}

func (s Stats) Summary() string {
	return fmt.Sprintf(
		"coast: %.0f km of shoreline, %.0f%% sea, shelf %.0f%% of it; surf planed %.1f km2 and cut %.2f Mm3,\n"+
			"       %d river mouths delivered %.2f Mm3, %.2f Mm3 laid (%.0f%% unplaced) as %.2f km2 of new beach;\n"+
			"       backshore %.0f m median, %.0f m P90, %.2f km2 drowned; exposure %.2f / %.2f / %.2f (p10/p50/p90)",
		s.ShorelineKm, s.SeaFraction*100, s.ShelfPctSea, s.PlanedKm2, s.CutM3/1e6,
		s.RiverMouths, s.RiverM3/1e6, s.LaidM3/1e6, pct(s.UnplacedM3, s.CutM3+s.RiverM3), s.BeachKm2,
		s.BackshoreM, s.BackshoreP90M, s.DrownedKm2, s.ExposureP10, s.ExposureP50, s.ExposureP90)
}

func pct(a, b float64) float64 {
	if b <= 0 {
		return 0
	}
	return a / b * 100
}

// Build lays the sea floor, cuts the shore and moves what it cuts. Height is modified in place.
func Build(in Input) *Result {
	h := in.Height
	w, ht := h.W, h.H
	cellArea := h.CellM * h.CellM

	g := Measure(in.Sea, w, ht, h.CellM)
	res := &Result{Geometry: g, Exposure: field.NewLike(h), Change: field.NewLike(h)}

	// Disabled, or a map with no coast on it: the sea floor is the flat plane at the abyssal depth, which is
	// what the generator produced before this pass existed. Everything below is skipped.
	if !in.Cfg.Enabled || len(g.Waterline) == 0 {
		for i := range in.Sea {
			if in.Sea[i] {
				h.Data[i] = float32(in.SeaLevelM - in.AbyssM)
			}
		}
		res.finish(h.Clone(), in)
		return res
	}

	shelfW := shelfWidth(h, g, in)
	layShelf(h, g, in, shelfW)

	// The before-and-after is taken here, after the sea floor and before the two shore processes. Taken any
	// earlier it would be a map of the sea floor: the ocean cells go from sea level to -180 m in one step, and
	// a few hundred metres of that swamps the few metres the surf and the sediment move, which is the thing
	// the map exists to show.
	before := h.Clone()

	shoreExposure := fetch(g, in)
	res.Stats.ExposureP10, res.Stats.ExposureP50, res.Stats.ExposureP90 = shorePercentiles(shoreExposure, g)
	carried := field.NewLike(h)
	for i, ref := range g.Ref {
		if ref >= 0 {
			carried.Data[i] = shoreExposure.Data[ref]
		}
	}
	// Smoothed for the same reason the shelf width is: carrying a per-shore value by "the stretch nearest to
	// you" partitions the map into Voronoi wedges, and a wedge boundary inside the deposition band would put
	// a straight edge through a beach.
	res.Exposure = boxMean(carried, int(exposureSmoothM/h.CellM+0.5), 2)

	cut := plane(h, g, res.Exposure, in)

	// The scatter into the supply array is serial and in index order on purpose: several land cells share a
	// waterline cell, so a parallel loop would be accumulating into the same slot from several goroutines and
	// the float sum would depend on who got there first. Cross-cutting rule 12 is not negotiable here, and
	// one linear pass over the grid costs nothing next to the solve.
	supply := make([]float64, w*ht)
	var cutM3, planedCells float64
	for i, c := range cut.Data {
		if c <= 0 {
			continue
		}
		v := float64(c) * cellArea
		supply[g.Ref[i]] += v
		cutM3 += v
		planedCells++
	}
	backshore, backshoreP90 := measureBackshore(h, g, in)

	riverM3, mouths := rivers(g, in, supply)
	laid, unplaced := deposit(h, g, res.Exposure, in, supply)

	res.Stats.CutM3 = cutM3
	res.Stats.RiverM3 = riverM3
	res.Stats.RiverMouths = mouths
	res.Stats.LaidM3 = laid
	res.Stats.UnplacedM3 = unplaced
	res.Stats.PlanedKm2 = planedCells * cellArea / 1e6
	res.Stats.BackshoreM = backshore
	res.Stats.BackshoreP90M = backshoreP90
	res.Stats.ShelfPctSea = shelfFraction(g, in, shelfW)
	res.finish(before, in)
	return res
}

// finish computes the mask the rest of the run should use, and the before-and-after difference.
//
// The mask is "strictly below sea level", not the continent mask it started from, and that is the point: a
// beach the pass built out of cliff debris is land, and a low headland it planed under the waterline is not.
// The statistics and the preview both ask what is above sea level, so they get an answer about the terrain
// rather than about the mask that seeded it.
func (r *Result) finish(before *field.Field, in Input) {
	h := in.Height
	r.Sea = make([]bool, len(h.Data))
	sea, beach, drowned := 0, 0, 0
	for i := range h.Data {
		r.Change.Data[i] = h.Data[i] - before.Data[i]
		r.Sea[i] = float64(h.Data[i]) < in.SeaLevelM
		if r.Sea[i] {
			sea++
			if !in.Sea[i] {
				drowned++
			}
		} else if in.Sea[i] {
			beach++
		}
	}
	n := float64(len(h.Data))
	cellArea := h.CellM * h.CellM
	r.Stats.SeaFraction = float64(sea) / n
	r.Stats.BeachKm2 = float64(beach) * cellArea / 1e6
	r.Stats.DrownedKm2 = float64(drowned) * cellArea / 1e6
	r.Stats.ShorelineKm = r.Geometry.ShoreM / 1000
}

// shelfWidth is metres of shelf for every cell: measured on the waterline from the relief standing behind it,
// carried out to sea by the nearest-shore reference, and then smoothed.
//
// The inland direction comes from the gradient of the signed distance field rather than from the eight-way
// step to the nearest land cell: the distance field is smooth, so the march does not stagger along the grid
// axes and the widths do not come out banded.
//
// The smoothing is not cosmetic. Carrying a per-shore quantity out to sea by "the stretch nearest to you"
// partitions the ocean into Voronoi wedges, and a wedge boundary is a discontinuity that runs for kilometres:
// the first render of the change map came out as a sunburst of straight rays radiating from every headland,
// which is a map of the feature transform rather than of a sea floor. Blurring the carried field over a few
// hundred metres turns the wedge boundaries back into what they should have been, a shelf whose width varies
// smoothly along the coast.
func shelfWidth(h *field.Field, g *Geometry, in Input) *field.Field {
	out := field.NewLike(h)
	steps := int(backshoreM/h.CellM + 0.5)
	lo := in.Cfg.ShelfKm.Lo() * 1000
	hi := in.Cfg.ShelfKm.Hi() * 1000
	steep := in.Cfg.SteepCoastM
	if steep <= 0 {
		steep = 1
	}
	field.Rows(len(g.Waterline), func(a, b int) {
		for n := a; n < b; n++ {
			i := int(g.Waterline[n])
			x, y := i%g.W, i/g.W
			dx := float64(g.Dist.AtClamped(x+1, y) - g.Dist.AtClamped(x-1, y))
			dy := float64(g.Dist.AtClamped(x, y+1) - g.Dist.AtClamped(x, y-1))
			l := math.Hypot(dx, dy)
			if l < 1e-6 {
				dx, dy, l = 1, 0, 1
			}
			dx, dy = dx/l, dy/l
			var relief float64
			for t := 1; t <= steps; t++ {
				px := x + int(math.Round(dx*float64(t)))
				py := y + int(math.Round(dy*float64(t)))
				if px < 0 || py < 0 || px >= g.W || py >= g.H {
					break
				}
				if e := float64(h.Data[py*g.W+px]) - in.SeaLevelM; e > relief {
					relief = e
				}
			}
			t := relief / steep
			if t > 1 {
				t = 1
			}
			out.Data[i] = float32(hi + (lo-hi)*noise.Smoothstep(t))
		}
	})

	carried := field.NewLike(h)
	for i, ref := range g.Ref {
		if ref >= 0 {
			carried.Data[i] = out.Data[ref]
		} else {
			carried.Data[i] = float32(hi)
		}
	}
	return boxMean(carried, int(shelfSmoothM/h.CellM+0.5), 2)
}

// layShelf writes the sea floor: a gentle shelf out to the break, then the continental slope to the abyss.
//
// The roughness is scaled by depth so it dies out at the waterline. Without that it puts metre-scale noise on
// water a few centimetres deep and the shallows come out as a scatter of one-cell islands, which then read as
// land in every statistic downstream.
func layShelf(h *field.Field, g *Geometry, in Input, shelfW *field.Field) {
	cfg := in.Cfg
	period := cfg.RoughWaveM * 256
	u, v := noise.WorldUV(g.W, g.H, h.CellM, 0, 0, period)
	rough := noise.FBMAt(u, v, noise.NewSource(in.Seed, srcShelf),
		noise.Params{BaseCells: 256, Octaves: 3, Gain: 0.5})

	exp := cfg.ShelfExponent
	if exp <= 0 {
		exp = 1
	}
	slopeW := cfg.SlopeKm * 1000
	if slopeW <= 0 {
		slopeW = 1
	}
	field.Rows(g.H, func(y0, y1 int) {
		for y := y0; y < y1; y++ {
			for x := 0; x < g.W; x++ {
				i := y*g.W + x
				if !in.Sea[i] {
					continue
				}
				d := -float64(g.Dist.Data[i]) // metres offshore
				width := float64(shelfW.Data[i])
				if width <= 0 {
					width = cfg.ShelfKm.Hi() * 1000
				}
				var depth float64
				if d < width {
					depth = in.BreakM * math.Pow(d/width, exp)
				} else {
					t := (d - width) / slopeW
					if t > 1 {
						t = 1
					}
					depth = in.BreakM + (in.AbyssM-in.BreakM)*noise.Smoothstep(t)
				}
				taper := depth / 10
				if taper > 1 {
					taper = 1
				}
				r := (float64(rough.Data[i])*2 - 1) * cfg.RoughnessM * taper
				h.Data[i] = float32(in.SeaLevelM - depth + r)
			}
		}
	})
}

// fetch is how open the water is in front of each waterline cell: rays cast seaward until they hit land,
// weighted by the cosine of their angle from the shore normal, and averaged.
//
// Two things about that sentence are the whole of it, and the first version got both wrong.
//
// **Only seaward.** Casting in every direction counts the land *behind* the shore as shelter, and every coast
// has land behind it — so a straight open coast, where seven rays in sixteen stop after one cell, scored as
// more sheltered than the back of a bay whose walls are half a kilometre off. Restricting to the half-space
// the shore faces, weighted by the cosine of the angle from the normal, is the standard effective fetch and it
// gets the sign right: open coast near 1, embayment well below it, enclosed inlet near 0.
//
// **Absolute, not a percentile.** The first version stretched the map's own 5th to 95th percentile onto 0..1,
// which is robust and which collapses to nonsense on a coast that does not vary — a perfectly straight one has
// no spread, so every cell of it came out at the same end of the scale and the whole continent read as one
// sheltered lagoon. A percentile is also a global statistic, which rule 1 of the tiling plan rules out: two
// tiles would stretch by different anchors and their shared bay would be two different colours. So the
// anchors are fixed and physical, and the units are "fraction of the fetch range the rays got".
func fetch(g *Geometry, in Input) *field.Field {
	out := field.New(g.W, g.H, g.CellM)
	dirs := in.Cfg.FetchDirections
	if dirs < 4 {
		dirs = 4
	}
	maxSteps := int(in.Cfg.FetchRangeM / g.CellM)
	if maxSteps < 2 {
		maxSteps = 2
	}
	cs := make([]float64, dirs)
	sn := make([]float64, dirs)
	for k := 0; k < dirs; k++ {
		th := 2 * math.Pi * float64(k) / float64(dirs)
		cs[k], sn[k] = math.Cos(th), math.Sin(th)
	}
	field.Rows(len(g.Waterline), func(a, b int) {
		for n := a; n < b; n++ {
			i := int(g.Waterline[n])
			x0, y0 := i%g.W, i/g.W
			// The seaward normal: the distance field increases inland, so its gradient points away from the
			// water and the negative of it is the direction this stretch of shore faces.
			nx := -float64(g.Dist.AtClamped(x0+1, y0) - g.Dist.AtClamped(x0-1, y0))
			ny := -float64(g.Dist.AtClamped(x0, y0+1) - g.Dist.AtClamped(x0, y0-1))
			if l := math.Hypot(nx, ny); l > 1e-6 {
				nx, ny = nx/l, ny/l
			} else {
				nx, ny = 0, 0 // no usable normal: fall back to the whole circle
			}
			var num, den float64
			for k := 0; k < dirs; k++ {
				w := cs[k]*nx + sn[k]*ny
				if nx == 0 && ny == 0 {
					w = 1
				} else if w <= 0 {
					continue
				}
				reach := maxSteps
				for t := 1; t <= maxSteps; t++ {
					px := x0 + int(math.Round(cs[k]*float64(t)))
					py := y0 + int(math.Round(sn[k]*float64(t)))
					if px < 0 || py < 0 || px >= g.W || py >= g.H {
						break // off the map is open water, and the mask keeps the border at sea
					}
					if !in.Sea[py*g.W+px] {
						reach = t
						break
					}
				}
				num += w * float64(reach) / float64(maxSteps)
				den += w
			}
			raw := 1.0
			if den > 0 {
				raw = num / den
			}
			t := (raw - shelteredFetch) / (openFetch - shelteredFetch)
			if t < 0 {
				t = 0
			} else if t > 1 {
				t = 1
			}
			out.Data[i] = float32(noise.Smoothstep(t))
		}
	})
	return out
}

// shorePercentiles reports the fetch distribution over the waterline itself, before it is carried anywhere.
func shorePercentiles(shore *field.Field, g *Geometry) (p10, p50, p90 float64) {
	if len(g.Waterline) == 0 {
		return 0, 0, 0
	}
	vals := make([]float64, 0, len(g.Waterline))
	for _, i := range g.Waterline {
		vals = append(vals, float64(shore.Data[i]))
	}
	sort.Float64s(vals)
	at := func(f float64) float64 {
		i := int(float64(len(vals)-1) * f)
		return vals[i]
	}
	return at(0.10), at(0.50), at(0.90)
}

// plane cuts the shore platform and returns how much it took off each cell, in metres.
//
// The shape is deliberate. Within the reach the land is planed nearly all the way to the platform, and only
// over the last quarter of the reach is the cut rolled off — so the profile is a gentle platform, then a short
// steep face, then untouched land. That face is the cliff. Rolling the cut off over the whole reach instead
// would give a ramp, which is what a coast looks like when someone has smoothed it rather than eroded it.
func plane(h *field.Field, g *Geometry, exposure *field.Field, in Input) *field.Field {
	cut := field.NewLike(h)
	cfg := in.Cfg
	field.Rows(g.H, func(y0, y1 int) {
		for y := y0; y < y1; y++ {
			for x := 0; x < g.W; x++ {
				i := y*g.W + x
				if in.Sea[i] {
					continue
				}
				d := float64(g.Dist.Data[i])
				e := float64(exposure.Data[i])
				reach := cfg.SurfReachM * (0.35 + 0.65*e)
				if reach <= 0 || d >= reach {
					continue
				}
				target := in.SeaLevelM + cfg.PlatformGrade*d
				above := float64(h.Data[i]) - target
				if above <= 0 {
					continue
				}
				w := 1.0
				if tail := reach * 0.25; d > reach-tail {
					w = noise.Smoothstep((reach - d) / tail)
				}
				residual := above * (1 - cfg.CutFraction)
				if residual > platformResidualCapM {
					residual = platformResidualCapM
				}
				c := (above - residual) * w
				h.Data[i] -= float32(c)
				cut.Data[i] = float32(c)
			}
		}
	})
	return cut
}

// measureBackshore is how high the land stands immediately behind the surf strip: between one and two surf
// reaches inland, so it is clear of everything the surf planed whatever the exposure there was.
//
// The median says what the ordinary coast is — a plain, on this continent, and it should be — and the P90 is
// the number that answers "are there sea cliffs anywhere on this map", which a median never can when most of a
// coastline is lowland.
func measureBackshore(h *field.Field, g *Geometry, in Input) (median, p90 float64) {
	lo := in.Cfg.SurfReachM
	hi := lo * 2
	vals := make([]float64, 0, 4096)
	for i := range in.Sea {
		if in.Sea[i] {
			continue
		}
		if d := float64(g.Dist.Data[i]); d < lo || d > hi {
			continue
		}
		vals = append(vals, float64(h.Data[i])-in.SeaLevelM)
	}
	if len(vals) == 0 {
		return 0, 0
	}
	sort.Float64s(vals)
	return vals[len(vals)/2], vals[int(float64(len(vals)-1)*0.9)]
}

// rivers adds each river mouth's load to the sediment supply. The load scales with what the river drains,
// sub-linearly, because the alternative is one trunk basin delivering more than every other mouth together.
//
// Only cells orthogonally against the water count as a mouth, so a channel contributes once or twice rather
// than along its whole lower course.
func rivers(g *Geometry, in Input, supply []float64) (total float64, mouths int) {
	if in.Flow == nil || in.Cfg.RiverM3PerKm2 <= 0 {
		return 0, 0
	}
	threshold := in.Cfg.RiverChannelKm2 * 1e6
	for i := range in.Sea {
		if in.Sea[i] || float64(g.Dist.Data[i]) > g.CellM*1.01 {
			continue
		}
		a := float64(in.Flow[i])
		if a < threshold {
			continue
		}
		v := in.Cfg.RiverM3PerKm2 * math.Pow(a/1e6, in.Cfg.RiverExponent)
		supply[g.Ref[i]] += v
		total += v
		mouths++
	}
	return total, mouths
}

// deposit carries the supply along the shore and lays it in sheltered shallow water.
//
// The transport is a box-kernel spread over DriftM, which is longshore drift at the only fidelity this grid
// can carry: it moves sediment out of the place it was cut and into the bays either side of it, and it does
// not pretend to know which way the waves run.
//
// # The order of the two operations, which is the whole of the mass balance
//
// Each source cell divides what it has among the cells around it in proportion to how much each wants it.
// Writing K for the kernel and w for the want, cell i receives
//
//	dep_i = w_i * sum_j K(i,j) * sup_j / Wbar_j,   Wbar_j = sum_k K(j,k) w_k
//
// which sums to exactly sum_j sup_j, because summing over i turns the inner weight back into Wbar_j. In code
// that is: divide the supply by the blurred want *first*, then blur, then multiply by the want.
//
// The obvious-looking alternative — blur the supply, then scale it by w_i / Wbar_i — is not the same thing and
// does not conserve. It was what this function did first, and it lost 68 % of the budget: the blur spreads
// supply onto land, onto deep water and onto exposed headlands, every one of which has w = 0 and is skipped,
// so everything that landed there was silently dropped. The test that caught it is an accounting identity, not
// a picture, which is the only kind of test that could have.
//
// The cap is the berm: nothing is laid more than BermM above sea level, because a beach crests and stops. What
// will not fit is offered once more to whatever still has room, and whatever is left after that is reported
// rather than dropped.
func deposit(h *field.Field, g *Geometry, exposure *field.Field, in Input, supply []float64) (laid, unplaced float64) {
	cfg := in.Cfg
	cellArea := h.CellM * h.CellM
	radius := int(cfg.DriftM/h.CellM + 0.5)
	if radius < 1 {
		radius = 1
	}

	// want is how much each cell of shallow, sheltered water will take. It is the only place sediment may go.
	want := field.NewLike(h)
	for i := range h.Data {
		if !in.Sea[i] {
			continue
		}
		if d := -float64(g.Dist.Data[i]); d > cfg.DepositReachM {
			continue
		}
		depth := in.SeaLevelM - float64(h.Data[i])
		if depth <= 0 || depth >= cfg.DepositDepthM {
			continue
		}
		shallow := (cfg.DepositDepthM - depth) / cfg.DepositDepthM
		shelter := shelterFloor + (1-shelterFloor)*math.Pow(1-float64(exposure.Data[i]), cfg.ShelterBias)
		want.Data[i] = float32(shelter * shallow)
	}
	norm := boxBlur(want, radius, 3)

	share := field.NewLike(h)
	for i, v := range supply {
		if v <= 0 {
			continue
		}
		nb := float64(norm.Data[i])
		if nb < 1e-9 {
			unplaced += v // nowhere within a drift length will take it
			continue
		}
		share.Data[i] = float32(v / nb)
	}
	spread := boxBlur(share, radius, 3)

	// place walks the grid in index order, which keeps the running totals deterministic: the writes are to
	// distinct cells but the sums are not, so this one stays serial.
	place := func(source *field.Field, scaled bool) (placed, over float64) {
		for i := range h.Data {
			w := float64(want.Data[i])
			if w <= 0 {
				continue
			}
			v := float64(source.Data[i])
			if scaled {
				v *= w
			}
			if v <= 0 {
				continue
			}
			room := in.SeaLevelM + cfg.BermM - float64(h.Data[i])
			if room <= 0 {
				over += v
				continue
			}
			dz := v / cellArea
			if dz > room {
				over += (dz - room) * cellArea
				dz = room
			}
			h.Data[i] += float32(dz)
			placed += dz * cellArea
		}
		return placed, over
	}

	laid, over := place(spread, true)

	// One more round for what would not fit, spread over the whole shore in proportion to want rather than
	// locally: by this point the material has already been carried as far as the model knows how to carry it.
	if over > 1 {
		var wsum float64
		for i := range h.Data {
			if want.Data[i] > 0 && float64(h.Data[i]) < in.SeaLevelM+cfg.BermM {
				wsum += float64(want.Data[i])
			}
		}
		if wsum > 0 {
			second := field.NewLike(h)
			for i := range h.Data {
				if want.Data[i] > 0 && float64(h.Data[i]) < in.SeaLevelM+cfg.BermM {
					second.Data[i] = float32(over * float64(want.Data[i]) / wsum)
				}
			}
			more, still := place(second, false)
			laid += more
			over = still
		}
	}
	return laid, unplaced + over
}

// shelfFraction is how much of the sea is shallower than the break, which is the number that says whether the
// margin came out as a shelf or as a trench with a rim.
func shelfFraction(g *Geometry, in Input, shelfW *field.Field) float64 {
	var shelf, sea float64
	for i := range in.Sea {
		if !in.Sea[i] {
			continue
		}
		sea++
		if -float64(g.Dist.Data[i]) < float64(shelfW.Data[i]) {
			shelf++
		}
	}
	if sea == 0 {
		return 0
	}
	return shelf / sea * 100
}

// boxMean smooths a *value* rather than a quantity: the same kernel, divided by how much of it landed on the
// grid, so a cell at the border keeps the average of its neighbours instead of being pulled towards zero.
//
// The distinction is not pedantic and it cost a test to notice. boxBlur is mass-preserving because it treats
// everything off the map as zero, which is right for sediment — there is none out there — and wrong for a
// shelf width, where off the map means "no information", not "a shelf of width zero". Smoothing the carried
// width with the mass-preserving kernel shrank every shelf near the border to nothing and put the whole
// margin below the break. Blurring a field of ones with the same kernel gives exactly the coverage to divide
// by, so the two share their arithmetic and cannot drift apart.
func boxMean(f *field.Field, radius, passes int) *field.Field {
	if radius < 1 || passes < 1 {
		return f.Clone()
	}
	ones := field.NewLike(f)
	ones.Fill(1)
	sum := boxBlur(f, radius, passes)
	cover := boxBlur(ones, radius, passes)
	out := field.NewLike(f)
	for i := range out.Data {
		if c := cover.Data[i]; c > 1e-6 {
			out.Data[i] = sum.Data[i] / c
		} else {
			out.Data[i] = f.Data[i]
		}
	}
	return out
}

// boxBlur is a separable running-sum box blur: O(n) whatever the radius, which is what makes a 300 m drift
// kernel cost the same as a 30 m one.
//
// It divides by the full window rather than by however much of the window was on the grid, which is to say it
// treats everything outside the map as zero. That choice is what makes the deposition sum come out right. The
// mass balance in deposit needs the kernel to be *symmetric* — a cell's share of its neighbour must equal the
// neighbour's share of it — and dividing each output by its own truncated window size breaks that symmetry at
// the border, which cost 4 % of the sediment budget on a coast that ran off the edge of the map. Zero padding
// keeps K(i,j) = K(j,i) everywhere, and a cell outside the map has no want, so nothing is owed to it.
func boxBlur(f *field.Field, radius, passes int) *field.Field {
	cur := f.Clone()
	if radius < 1 || passes < 1 {
		return cur
	}
	inv := 1 / float64(2*radius+1)
	next := field.NewLike(f)
	for p := 0; p < passes; p++ {
		field.Rows(f.H, func(y0, y1 int) {
			for y := y0; y < y1; y++ {
				row := y * f.W
				var sum float64
				for x := 0; x <= radius && x < f.W; x++ {
					sum += float64(cur.Data[row+x])
				}
				for x := 0; x < f.W; x++ {
					next.Data[row+x] = float32(sum * inv)
					if hi := x + radius + 1; hi < f.W {
						sum += float64(cur.Data[row+hi])
					}
					if lo := x - radius; lo >= 0 {
						sum -= float64(cur.Data[row+lo])
					}
				}
			}
		})
		cur, next = next, cur
		field.Rows(f.W, func(x0, x1 int) {
			for x := x0; x < x1; x++ {
				var sum float64
				for y := 0; y <= radius && y < f.H; y++ {
					sum += float64(cur.Data[y*f.W+x])
				}
				for y := 0; y < f.H; y++ {
					next.Data[y*f.W+x] = float32(sum * inv)
					if hi := y + radius + 1; hi < f.H {
						sum += float64(cur.Data[hi*f.W+x])
					}
					if lo := y - radius; lo >= 0 {
						sum -= float64(cur.Data[lo*f.W+x])
					}
				}
			}
		})
		cur, next = next, cur
	}
	return cur
}